1. Field of the Invention
This invention relates to a method of patterning a resist layer, and more particularly to a method of forming micro patterns.
2. Description of the Related Art
A conventional method of patterning a material layer will be described by referring to FIGS. 1A-1C. As shown in FIG. 1A, a material layer denoted as 15, for example, an SiO.sub.2 film, is formed on semiconductor substrate 14 of, for example, silicon Resist layer 16 is formed on material layer 15. As shown in FIG. 1B, an exposing device is used, and resist layer 16 is irradiated with light rays projected through a mask pattern or a reticule (not shown) having a predetermined pattern. The resultant structure is subjected to a development process, to form a resist pattern as shown in FIG. 1C. Further, material layer 15 is etched with a mask of the resist pattern. In this way, material layer 15 is patterned.
When using the conventional patterning method as mentioned above, a cross sectional configuration of the side wall of the developed resist pattern varies with respect to the kind of resist material, developer, and the type of the exposing device used. It is, therefore, very difficult, to obtain a resist pattern having a desired side wall shape. Ideally, the side wall of the resist pattern should be orthogonal to the surface of semiconductor substrate 14. In the case of a resist pattern type wherein a difference between the solubility of the exposed part of the pattern and that of the unexposed part thereof is small, the configuration of the side wall of resist pattern 16 is shaped such that when viewed in the drawing, the configuration flares upwards, as shown in FIG. 1C. In other words, the cross sectional shape of the side wall resembles a reversed trapezoid. This resultant shape of the side wall is due to the fact that the amount of exposing light which reaches the edges of the etched portion of resist layer 16 is smaller than the amount of exposing light which reaches the central part thereof, and the fact that the amount of exposing light decreases towards the deep portion of resist layer 16. Particularly, in forming a micro pattern, it is difficult to form a resist pattern having a side wall which is orthogonal to the surface of semiconductor substrate 14. When the lift-off process is used for the patterning of the resist layer, it is desirable that the side wall of the resist layer be shaped to have an overhang If the side wall of the resist layer is so shaped, it will be easier to disconnent a metal layer to be later layered over the resist layer. The conventional patterning method as mentioned above, however, cannot form the resist pattern so as to have such an overhanging side wall.
As described with reference to FIGS. 1A to 1C, the conventional method of forming micro patterns includes the process steps of resist coating, exposure, and development, which are executed in this order. In the conventional method, the shape of the side wall of the resist pattern varies with respect to the kind of resist material, developer, and the type of the exposing device used. This makes it difficult to form resist patterns having side walls of a desired shape.